late 14c. metathesis of thritti, from Old English þritig, from þri, þreo "three" (see three) + -tig "group of ten" (see -ty (1)). Similar formation in Old Frisian thritich, Old Saxon thritig, Dutch dertig, Old High German drizzug, German dreissig.
The Thirty Years' War (1842) was a general European religious and dynastic power struggle, furiously destructive, waged 1618-48, mainly on German soil. The symbol -30- as printer and telegrapher's code to indicate the last sheet or line of copy or a dispatch is recorded from 1895. In 20c. jargon of newspaper journalism, it came to be a traditional sign-off signal and slang word for "the end."